The present invention relates to a device to compensate for the unbalance of incident light intensities transmitted by two rangefinding optical systems provided in an automatic focus adjusting system and the image size differences (or the light intensity differences of the images) formed on photosensitive elements which is caused by manufacturing tolerances of the lenses provided in the optical paths of the two optical systems by adding one more lens element in one of the two optical paths (intensity differences are compensated for by light attenuation).
The compensating technique disclosed herein to remove the magnification differences by focal length differences of lenses by adding one more lens element is well-known, but the application of this technique to a rangefinding autofocus system is a unique recognition of the problem and its solution. Of the many devices for adjusting focus automatically, each has its functional limits. A few specially designed systems work for special purposes, but because of the aforesaid limitations errors can occur in adjusting focus or measuring the subject distance.
Improvements are continuously being made to optical instruments such as cameras having a focusable objective lens. One of the more recent series of improvements is an automatic focusing system for adjusting the focus of the lens to an object distance corresponding to the distance of the camera from remote subjects in the field of view of that lens. Typically, an automatic focusing system may use the principle of spatial image correlation wherein a scene imaged by a first auxiliary optical system is scanned by a sensor and the image therefrom is correlated electronically by another sensor with an image from a fixed auxiliary optical system. Such an automatic focusing system has electronics for evaluating the light bundles from images passed through those optical systems and impinging on sensor arrays. Electronics are used to control power for a motor for driving the focusing cell of an objective lens in accordance with the relationship of the images transmitted by the optical systems. Focus of the objective lens occurs when the images from the optical systems are identical; under such conditions the position of the scanning optical system is related to the position of the objective lens.
Certain of the automatic focusing systems, as described in recently filed (see Ser. No. 874,587, assigned to the same assignee as the present application) and issued United States patents, can be mass produced, are neither unduly complex, bulky, nor expensive, and are reliable for use in optical instruments or cameras for the mass market. Such systems include a scanning means, which moves an optical axis to sweep an image across a light sensitive array, and which is connected with another similar array of a conjugate detector. One of the optical axes is fixed relative to the rangefinder and camera lens and the other is movable so as to scan along the other optical axis. An automatic rangefinder array of the type generally described in U.S. Pat. No. 4,002,899, assigned to Honeywell, Inc. and called the Honeywell Visitronics Module, responds to the two optical systems that transmit light from the subject to a detector having a pair of photosensitive arrays. The module is responsive to the fact that the axes of the optical systems are aligned with one another and directed toward the same subject. Such alignment causes the detector to generate a peak pulse, which pulse can be used to adjust the focus of the camera lens. Rangefinding is accomplished by triangulation and comparison of the image from the fixed axis with the image from the scanning axis. By means of suitable electronic circuitry, the peak signal can be converted to a servo signal for the focusing motor.